Information communication equipment, especially compact personal equipment such as mobile terminals have become very popular and this raises a strong need for higher performances such as higher integration, higher speed, lower power consumption, etc. of memories, logic devices and the like to be used in such equipment. In particular, non-volatile memory devices are considered to be essential in the ubiquitous age. In case of power shortage or a trouble of the equipment, or even in case of interruption of the connection due to a trouble between the server and the network, the non-volatile memory devices are able to hold the stored information and thus protect important information.
Currently, flash memories using semiconductor, FRAMs (Ferro-electric Random Access Memories) using ferro-electric material and the like are widely used as such non-volatile memory devices. However, flash memories have a disadvantage in that the writing speed is in the order of microseconds and thus very slow. On the other hand, FRAMs can be rewritten in the range of 1012-1014, thereby making them impossible to completely replacing SRAMs (Static Random Access Memories) or DRAMs (Dynamic Random Access Memories) because of durability. Moreover, difficulty in microminiature processing of FRAMs is pointed out.
Incidentally, more recent portable equipment are designed to maintain non-used circuit blocks in stand-by condition for reducing unnecessary power consumption as small as possible. However, if a non-volatile memory having both functions of a high speed work memory and a large capacity storage memory is achieved, it is possible to avoid waist of power and memories. It is also possible to realize so-called instant-on function in which the system can be started immediately after power-on if a high speed and large capacity non-volatile memories are available.
Non-volatile memories that may achieve the above objectives are attracting a great deal of attention and include, for example, a magnetic memory that is known as a MRAM (Magnetic Random Access Memory) (See Wang et. al., IEEE Trans. Magn. 33 (1997), 4498). The MRAM device is a semiconductor magnetic memory utilizing magneto resistance effect based on the spin dependent conduction phenomenon that is peculiar to a nano magnetic material and is a non-volatile memory capable of holding stored information without externally supplying power.
Information writing in the MRAM device is carried out by combined magnetic fields of bit lines and word lines wired in matrix for inverting the magnetic spin of the cell at the cross point of the bit line and the word line, thereby recording the orientations as the information “1” or “0”. On the other hand, information reading is carried out using the TMR (Tunnel Magneto Resistance) effect to which the magneto resistance effect is applied. The TMR effect is the phenomenon in which the resistance changes depending on the orientation of the spin. In case of reading the MRAM device, the information “1” or “0” is read out depending on whether the resistance is high or low.
Additionally, since the MRAM device has a simple construction, it is easy to integrate. Also, since recording is made by rotating the magnetic moment, the number of rewritings can be very large. The access time is forecasted to be very high and is reported to be operable at 100 MHz (See R. Scheuerlein et. al., ISSCC Digest of Technical Papers, pp. 128-129, February 2000). As understood from the above, in conjunction with improved performance of the TMR materials in recent years, the MRAM devices are attracting a great deal of attention and there are increased hopes as high speed and large capacity non-volatile memories.
However, since the MRAM devices use a magnetic material for recording and holding the information, there is a possibility that the recorded information is erased or rewritten by the influence of external magnetic fields. Normally, the MRAM devices are mounted on a circuit board inside the electronic machine or equipment. Depending on kinds of electronic machine or equipment, as a progress of highly density mounting in recent years, mounted densely on a circuit board are various devices such as semiconductor devices, communication devices, miniature motors and the like other than the MRAM devices. Various mechanical components including an antenna, a power supply unit and the like are also installed inside the electronic machine or equipment to constitute a single apparatus together with the circuit board having the MRAM devices mounted thereon. This means that the MRAM devices are used in an environment of mixed magnetic fields over a wide frequency range including DC, low frequencies up to high frequencies. Accordingly, in order to put the MRAM devices into practical use, the MRAM devices need to be improved in resistance to external magnetic fields by means of improved mounting and/or magnetic shielding of the MRAM devices, thereby enhancing reliability of holding the recorded information in such devices.
As examples of magnetic shielding of such MRAM devices, U.S. Pat. Nos. 5,902,690 and 5,929,772 propose magnetic shielding structures for MRAM devices. In particular, U.S. Pat. No. 5,902,690 proposes to provide the MRAM device with a magnetic shielding characteristic by using insulation ferrite (MnZn and NiZn ferrites) layers that are soft magnetic metal oxides as a passivation film of the MRAM devices. On the other hand, U.S. Pat. No. 5,929,772 proposes to form a permalloy which is a magnetic member having a high magnetic permeability inside a package, thereby preventing magnetic flux from penetrating inside the device.
However, such conventionally proposed shield structure for MRAM devices is not yet perfect and has a possibility to cause a problem by permitting magnetic flux to penetrate into the MRAM devices and to erase the stored information or overwrite.
In order to prevent magnetic flux from penetrating, it is necessary and the most important to dispose a magnetic material having a high permeability about the device, thereby providing a magnetic path so that magnetic flux does not penetrate into the device. In this point of view, the conventional methods to form a ferrite passivation film or forming a permalloy inside the package provide an incomplete magnetic shield structure because the magnetic path is not completely closed.
Moreover, in case of using a ferrite layer as the passivation film that is made by a film forming by the sputtering technique, since the ferrite is an oxide magnetic material, it is most likely to cause oxygen loss, thereby making it very difficult to use a complete ferrite layer as the passivation film. On the other hand, if the ferrite layer is formed by any technique other than sputtering, there encountered other problems such as increased manufacturing steps and cost in addition to decreased production yield.
Furthermore, in case of disposing permalloys at both top and bottom locations within the package, it is possible to obtain better shielding performance than the ferrite passivation film. However, since there is no magnetic interconnection between the top and bottom permalloys, the magnetic path is not established. Although this technique is effective in high frequencies, a large effectiveness is not expected for low frequency magnetic fields that are common to the MRAM devices. Even if one of the top and bottom permalloys is jointed to a ground layer of the device using an electrically conductive adhesive, the above mentioned drawback of not establishing the magnetic path is still applicable, thereby not providing significant magnetic shielding.
The present invention is made in consideration of the above circumstances and it is an object of the present invention to provide a non-volatile magnetic memory device that provides sufficient magnetic shielding performance for external magnetic fields over a wide frequency range.